kerneldump: add livedump_start_vnode(9)

livedump_start_vnode(9) is introduced such that the live minidump on the
system could take a vnode. This interface could be used to extend support
for the existing framework in downstream.

Bump __FreeBSD_version for introducing livedump_start_vnode(9).

Sponsored by: Juniper Networks, Inc.
Reviewed by: khng
Differential Revision: https://reviews.freebsd.org/D43471